Cellular differentiation of multicellular organisms is controlled by hormones and polypeptide growth factors. These diffusable molecules allow cells to communicate with each other and act in concert to form tissues and organs, and to repair and regenerate damaged tissue. Examples of hormones and growth factors include the steroid hormones, parathyroid hormone, follicle stimulating hormone, the interferons, the interleukins, platelet derived growth factor, epidermal growth factor, and granulocyte-macrophage colony stimulating factor, among others.
Hormones and growth factors influence cellular metabolism by binding to receptor proteins. Certain receptors are integral membrane proteins that bind with the hormone or growth factor outside the cell, and that are linked to signaling pathways within the cell, such as second messenger systems. Other classes of receptors are soluble intracellular molecules.
In general, there is conservation among the cytokine receptors, which are classified into two classes. The cytokine ligands for these receptors have some conserved structural identity, as well, however, the biological activity of these ligands is diverse. The molecules of the present invention belong to a structural class of ligands that is characterized by comprising a four-helical bundle. As a group, the cytokine family of ligands have been extremely valuable as therapeutics and reagents for understanding the growth and maturation of many cell types. The cells influenced by cytokines range from totipotent stem cells to terminally differentiated cells with specialized functions critical to homeostasis of a broad spectrum of living systems. Thus, based on the activities of the cytokine family of proteins there is a need for new cytokines, cytokine agonists and cytokine antagonists, as well as related compounds and methods. The present invention provides such polypeptides for these and other uses that should be apparent to those skilled in the art from the teachings herein.